Process of treating aluminum



Reiaeued May 16; .1933

UNITED STATES PATENT OFFICE um I. cmmx, or rrr'rsrrmm, uassacnusn'r'rs, Assmuon. "r0 GENERAL ELECTRIC courm, A coarom'rron or NEW You access or 'ram'rme summon Io Drawing. Original No. 1,846,844, dated February 23, 1982, Serial Io. 378,820, fled July 18, 1929.

Application for reissue filed April 4, 1933 Serial No. 664,444..

The present invention relates to a treatment ofaluminum which has the object in view of producing a strongly adherent dielectric coating on the aluminum. Itis a particular object of m invention to produce on aluminum foil a dielectric coating which will render the coated aluminum foil suitable for use as an electrode in condensers without the necessary interposition of any other dielectric material..

Numerous methods have been suggested heretofore whereby aluminum may be provided with a dielectric coating, as, for example, by an electrolytic treatment in which the aluminum to be treated functions as an electrode of an electrolytic cell. The coating formed by the electrolytic method is not suit able for condenser dielectric It has also been suggested to produce colored eflects upon the aluminum by chemical treatment without the application of externally applied electric current, that is, without electrolysis. Such coatings, however, as heretofore produced have been too thin and of too low insulation value to be suitable for use as condenser dielectrics or other insulating pur- I as a retarder which modifies 'the otherwise too violent efl'ect of the alakli.

The nature of the insulating'coating which is formed in accordance with my invention has not been determined with complete certainty but insofar as its nature has been determined the evidence indicates that it consists largely of oxide of aluminum together with other oompoundsof aluminum depending on the nature of the electrolyte. For

example, when asilicate of an alkali is used .as a retarder some silicate of aluminum is formed. In any event, properties of alumimun treated in accordance with my inven- In accordance with my invention a'hard,

tion are especially poses. a y

In carrying out my invention the foil, or other article'of aluminum, which is to be rovided with a dielectric coating is subected to the chemical action of an alkaline reagent which maybe prepared as below described; without the application of any externally applied electrical energy. The length of time of treatment varies inaccordance with the results to be produced and in accordance with the concentration of the reagent, the temperature and other conditions. In'general the temperature is maintained at about 65 to 75 C. The aluminum is maintained in contact with the alkali agent under average conditions for periods up to 13 minutes. Q i I An alkali solution suitable for carrying out my invention may contain about 2 to 6% by weight of sodium hydroxide, about 1.5 to 3% of ammonia (NH,) and about 2.5 to 12% of a 'retarder which preferably consists of sodium silicate. The ammonia may be omitted in some cases and in any event it is not requisite that the proportion of the ammonia should be maintained within the particular values given'above. In place of sodium hydroxide other alkali reagents may be employed, for example, the hydroxides of other metals of the alkali rou ,such as potassium hydroxide or a hy roxide of the alkaline earth grou as, or example, calcium hydroxide. ollectively such hydroxides are termed strongly alkaline metal base hydroxides. In place of the sodium silicate other modifying reagents may be employed, as, for example, the tetraborate, fluoride or carbonsuited for dielectric purate of sodium or other alkaline metal or the be varied as already indicated by the above range of percentages. When bringing an aluminum foil, wireor other article into contact with the alkaline reagent by conducting such aluminum body in continuous lengths through the alkali reagent, I prefer to em- I The dielectric film produced in accordance with myinvention is very fine grained and adherent. It resists electrical breakdown upon the application of voltages up to about 3,000 volts. It has a power factor .better than .5%. Theinsulation resistance of a 1 microfarad condenser containing aluminum foil treated in accordance with my invention is of the order of magnitude of 6,000 megohms. The electrical capacity of the film varies, of course, with its thickness, but as an illustrative example I may state that a film about.3 to .5 mil in thickness and having an area of about 3 square feet has a capacity of about 1 microfarad.

It is not necessary, nor even desirable, to

a wash and dry aluminum foiltreated in ac- What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The process of forming an electrical insulating coating on aluminum which consists in bringin the same without the application of vex terna ly applied electrical energy into contact with an aqueous solution of a silicateof .an alkali metal, said silicate being associated with hydroxide of alkali metal in suflicient quantity to produce by chemical action a dielectric coating on said aluminum.

2. The process'of oxidizing aluminum which consists in acting on said metal without the application of externally applied electrical-energy with a solution of an hydroxide of an alkaline metal and a salt-of said metal whereby the chemical effect of said hydroxide on said aluminum is retarded, the'latter ingredient being in preponderant proportion by weight-.

3, The process of treating aluminum which consists in bringing the same into contact without the application of externally applied electrical energy with an aqueous solution' containing about 2.5 to 12 per cent of a silicate of an alkali metal, and about 2 to 6 per cent of an alkali metal hydroxide.

4. The process of producing an adherent, insulating coating on aluminum which consists in immersing the aluminum to be coated. without the application of externally applied electrical energy in a solution containing by weight about 2.5 to 12% sodium silicate, about 2 to 6% of sodium hydroxide and about 1.5 to 2.5% ammonia.

5. The process of treating aluminum which consists in bringing said metal without the application of externally applied electrical energy into contact with an a ueous solution containing a silicate of an a ali metal and hydroxide of calcium.

6. The process of treating aluminum to produce an insulating coating thereon which' consists in bringing said metal withoutthe' application of externally applied electrical energy into contact with an aqueous solution containing a salt ofan alkaline metal and hydroxide of calcium.

7. The process of producing an adherent,- insulating coating on aluminum whichcon sists in immersing aluminum to be coated. in

a solution containing both a hydroxide and-a salt of an alkaline base without application of externally applied electrical energy. u

8. The process of dielectric coating on a uminum which consists in treating the same without the application roducing an adherentof externally applied electrical energy with a an aqueous solution containing a strongly alkaline metal base hydroxide, amm0nia,- anda substantial amount of a salt'of an alkali metal having a chemical retarding effect.

In witness whereof, I have hereunto set in hand.

y FRANK. M. CLARK. 

